The present invention relates to an image forming apparatus, in which a latent image is formed on a photosensitive body by using the electrophotographic process and developed by means of toner particles, and the developed image is delivered onto a sheet of paper for use as a transfer medium, and a developing device adapted for use in the image forming apparatus.
In an image forming apparatus that utilizes the electrophotographic process, an electrostatic latent image is formed by giving a predetermined potential to a photosensitive body having photoconductivity, applying light corresponding to image information to the photosensitive body, and selectively attenuating the potential of the photosensitive body, and toner particles are fed to the latent image, whereupon a copy image i.e., printable image of an object of copying is outputted.
The toner particles fed to the photosensitive body i.e., the resulting toner image is transferred to a sheet of paper for use as a transfer medium, and fixed to the sheet of paper by means of a fixing device. Untransferred toner particles remaining on the photosensitive body are removed from its surface by means of a cleaning device.
Many of copying apparatuses use a method in which the toner particles members are frictionally charged to the full by means of carrier members, and the electrostatic latent image formed on the photosensitive body is developed by being fed with the frictionally charged toner particles by means of a developing roller that is located at a fixed distance from the surface of the photosensitive body.
In this case, the quantity of the toner particles attached to the latent image, that is, image density, is maintained by moving (or rotating) the outer peripheral surface i.e., a developing sleeve of the developing roller at a speed higher than the moving speed of the surface of the photosensitive body.
If the developing sleeve of the developing roller is rotated at high speed, however, then the toner particles will be scattered around the photosensitive body or in the copying apparatus.
This scattering is caused by insufficiently charged toner particles, that is, low-charged toner particles. The force of electrostatic attraction between the low-charged toner particles and the carrier members is smaller than that between the normal toner particles and the carrier members. If the developing sleeve of the developing roller is rotated at high speed, therefore, the low-charged toner particles are scattered as it is released from the electrostatic attraction to the carrier members by centrifugal force.
This toner particles scattering can be prevented by two methods, improvement of the developing agent and improvement of the developing device.
According to an example of the method in which the developing agent is improved, the amount of frictional charge on the toner particles are increased to augment the force of electrostatic attraction between the toner particles and the carrier members. Although scattering of the toner particles can be prevented, according to this method, the amount of frictional charge on the toner particles is so large that a high image density cannot be obtained with ease.
According to a proposed example of the method in which the developing device is improved, the ratio of the moving speed of the surface of the developing sleeve of the developing roller to the speed at which the moving speed of the outer peripheral surface of the photosensitive body (hereinafter referred to as processing speed), is reduced. The moving speed of the surface of the developing sleeve can be lowered by increasing the outside diameter of the sleeve. Accordingly, the centrifugal force to which the toner particles on the sleeve is subjected can be reduced by increasing the diameter of the sleeve.
However, the increase of the diameter of the developing sleeve results in an increase in size of the developing device, thereby making the copying apparatus large-sized. Thus, the total cost of the apparatus increases inevitably.
In consideration of these circumstances, there has recently been proposed a developing method that uses small-particle carrier members.
With use of the small-particle carrier members, the specific surface area of a carrier particle compared with the toner particles can be increased. With the ratio in weight between the toner particles and the carrier members is fixed, therefore, the toner concentration can be set at a high value. This indicates that the developing efficiency can be improved. In the case where the target image density is fixed, the increase of the developing efficiency can make the rotating speed of the developing sleeve lower than in the conventional case, thus helping the reduction of the toner particles scattering.
It is ascertained, however, that the small-particle carrier members, especially one with a particle diameter of 50 .mu.m or less, the carrier members adheres to the photosensitive body (so called carrier adhesion) is occurred.
Thus, although the small-particle carrier members can improve the developing efficiency, it is of no practical use on account of its tendency to adhere to the photosensitive body.